In recent years, there has been an increasing demand for the development of imaging lens systems for surveillance video cameras used for monitoring from a long distance. To monitor from a long distance, zoom lenses with large zoom ratios are required. However, it is difficult for the zoom lenses to achieve favorable properties over the entire zoom range. In order to achieve these favorable properties, the sizes of the zoom lenses tend to become large.
Zoom lenses with large zoom ratios suitable for surveillance video cameras are described in Japanese Laid-Open Patent Application 2003-107345 as “zoom lens systems with very large zoom ratios” and provide zoom ratios of approximately fifty-five. Each of these lens systems includes, in order from the object side, a first lens group having positive refractive power, a second lens group having negative refractive power, a third lens group having negative refractive power, and a fourth lens group having positive refractive power. The second and third lens groups move for zooming in order to vary the focal length of the zoom lens and to compensate for movement of the image plane with the variation in focal length.
In contrast, as zoom lenses with high quality used for broadcast television cameras, zoom lenses that include, in order from the object side, lens groups having negative and positive refractive powers that move for zooming have been conventionally and widely used as zoom lenses of large size and with large zoom ratios. These zoom lenses include, in order from the object side, a first lens group that is fixed when zooming, a second lens group having negative refractive power that is movable when zooming, a third lens group having positive refractive power that is movable when zooming, and a fourth lens group that is fixed when zooming. In these types of zoom lenses, the second lens group acts primarily as the variator for varying the focal length of the zoom lens and the third lens group acts primarily as the compensator for compensating for movement of the image plane that would otherwise occur with variation in the focal length due to movement of the second lens group. However, the third lens group is capable of contributing variations in the focal length by its movement to some extent, and therefore the moving distance of the second lens group becomes relatively shorter, which results in shortening the overall length of the lens system.
An example of this type of zoom lens is described in Japanese Patent No. 3486560. As described in Japanese Patent No.3486560, this type of zoom lens includes, in order from the object side, a first lens group having positive refractive power, a second lens group having negative refractive power, a third lens group having positive refractive power, and a fourth lens group having positive refractive power; and a lens element of the third lens group includes an aspheric surface. This type of zoom lens is described as having an f-number of approximately 1.5–1.7, which is a relatively large aperture ratio, and a zoom ratio of approximately nineteen to fifty, which includes large zoom ratios.
However, the demand for a zoom lens having a larger zoom ratio of approximately sixty is expected to be required soon in the surveillance camera market. In addition, the major application of surveillance video cameras used in harbors is for monitoring from a long distance and high performance at close range is not as necessary as with applications of standard television cameras for broadcasting, and the zoom lenses of Japanese Patent No. 3486560 are described as high performance zoom lenses for use in television cameras for broadcasting. Moreover, there is a tendency to increase the size of the lens elements placed on the object side in order to provide appropriate performance at close range. Therefore, the need for the development of a comparatively compact zoom lens with a large zoom ratio suitable for mounting in surveillance video cameras is clear.